'The Art of Celtic Knots' printed from http://nrich.maths.org/

About 1200 years ago Irish Monks worked for years to create a
masterpiece in 'illuminated' script, called the Book of Kells . This ancient book is now kept in
Trinity College Library in Dublin.

Look closely at this piece of one page (above) and you will see a
Celtic Knot design inside the huge fancy letter. This type of
design has been studied by many people. Some people like Celtic
Knots for artistic reasons, others are fascinated by the
mathematics of the designs. More pictures can be found at Book of Kells Images.

Many Celtic knots are continuous - as if they are made from one
long piece of string. So they are 'mathematical' knots. Others are
made up of sections, with each section made so that if you trace
along the line you come back to where you started. So these are
'links' of mathematical knots.